The present invention relates to sized paperboards. More particularly, the invention relates to novel and improved multi-ply paperboards useful in the manufacture of gypsum wallboards.
Gypsum wallboard is a well known structural precast unit useful as the wall or ceiling material of residential or industrial buildings and made of a gypsum core which has been set by hydration and two covering multi-ply paperboards which sandwich the core, the contacting surfaces being firmly bonded to each other.
Such gypsum wallboards are manufactured, according to the most widely practiced process, in the following steps or operations. An aqueous hydraulic slurry of calcined gypsum is poured into the space provided between two separate multi-ply paperboards while continuously and endlessly advancing at the same velocity. As the gypsum slurry becomes to set or hardened due to hydration to form a core sandwitched by the two covering paperboards, the whole board is passed through a high-temperature drying kiln, where most of excessive water content in the board is removed by evaporation. The thus treated board is cut into desired lengths.
The paperboard, specifically the core-side liner or ply of the multi-ply paperboard, can bond to the hardened gypsum core without the use of any adhesives in principle. This is because numerous needle-like cystals are formed in the gypsum slurry soaked in the paperboard and elongate into the texture of the paperboard, resulting in an intimately interlaced structure to produce a sufficient bonding strength between the gypsum core and the covering paperboard.
It is a conventional technique to add to the aqueous slurry of calcined gypsum small amounts of a water-soluble polymeric substance, such as starch. The addition of starch is intended, in part on one side, to produce an auxiliary adhesive bond between the gypsum core and the paperboards but, in major part on the other side, to provide coatings on the crystals of the hydrated gypsum so that any losses in bonding strength between the paperboards and the hydrated gypsum core can be prevented if and when the crystals of the hydrated gypsum (CaSO.sub.4.2H.sub.2) is dehydrated into the state of calcined gypsum (CaSO.sub.4.1/2H.sub.2) or further into the state of anhydrous gypsum (CaSO.sub.4) during the drying step in the high temperature kiln operated at excessively high temperatures, say, above 80.degree. C.
Important technical problems to be solved in the above-described conventional manufacturing process of gypsum wallboards include the following:
(1) The drying velocity in the drying kiln should be sufficiently high to ensure high productivity.
(2) The interlacing of the hydrated gypsum crystals and the paper texture should be well developed so as to give a sufficient bonding strength.
(3) The amount of an expensive water-soluble polymeric substance like starch to be added to the aqueous slurry of calcined gypsum should be reduced to as low as possible without causing troubles with respect to the problems (1) and (2) above.
(4) It should be realized that the starch added does not spread evenly throughout the inside and surface of the hydrated gypsum core or migrate into the entirety of the multiplied paperboards, but concentrate near the interface between the core of the hydrated gypsum and the covering paperboards.
The solution of the above problems is largely dependent on the quality of the paperboards used. For the purpose, the paperboards are required to have such qualities as high mechanical strengths, low moisture absorption, small changes in dimensions when wet, and fine appearance as well as adequate water absorptivity and high air permeability, the latter two qualities being particularly important. For example, if the air permeability of the paperboards is not sufficiently high, the dissipation of water vapors during the drying ste is hindered, and it is required disadvantageously to provide a longer drying kiln.
The water absorptivity and air permeability are, sometimes, contradictory requirements to each other for a paperboard suitable for the manufacture of gypsum wallboards. It is a very difficult problem to satisfy both requirements simultaneously. For example, conventional sizing materials, such as rosin-alum, natural waxes, acrylic resins, and the like, which are used for the purpose of decreasing the water absorptivity of the paperboards, work to remarkably reduce air permeability and, for this reason, can not be suitable for sizing paperboards to manufacture gypsum wallboards.
A method has been proposed in the prior art to solve the above-described technical problems encountered in the manufacture of gypsum wallboards, in which the paperboards are treated in advance with certain silicone resins, e.g. an epoxy-modified silicone resin (see, for example, U.S. Pat. Nos. 3,389,042 and 3,431,143). The method, however, is disadvantaged by the following reasons, and can not be satisfactory from the practical point of view.
(1) That certain expensive silicone resins are used in relatively large amounts.
(2) That the paperboards as treated with a silicone resin have to be stored for many days in accumulation before the silicone resin is sufficiently cured and the paperboards are put to processing for the manufacture of gypsum board product.
(3) That the paperboards as finished tend to have non-uniform quality due to local variations in the degree of curing, since the curing reaction of silicones is very susceptible to conditions under which the silicone-treated paperboards are stored.
In addition to the above technical problems which are principally concerned with the bottom liner ply of the multi-ply cover paper directly adjacent the gypsum core, similar problems are encountered with respect of the outermost ply or top liner ply exposed and not in contact with the gypsum core. For example, when a sufficient sizing effect is intended using conventional sizing agents, a great deal of sizing is necessitated and, as a result, not only the air permeability of the resulting paperboard will be lost to an extent inadequate for processing into gypsum wallboards, but also the resistance to moisture absorption, which is also a very desirable property for the finished gypsum wallboard, will not be expected. Therefore, such gypsum wallboards are met with further problems, such as the possibility of the top liner ply to peel during transportation or during secondary processing, e.g. surface finishing, and the intolerable degradation of quality by moisture absorption during storage.